I made a conscious decision to go from the stock-specified 13 pound cap to a 16 pound cap, in order to keep it from boiling over just a little bit longer. If my cooling system was behaving, I would've stayed with the 13 pound cap.
From discussions here and elsewhere on the subject of cooling system pressure, it seemed like the systems (radiator, heater core, etcetera) could handle well above 20 pounds, so 16 seemed pretty safe to me.
IIRC my book calls for 14lb cap but you can't find them so I use 13. Those caps work under a range of pressures. 13lb could be 12-14, a 16lb cap 14-18; something like that. I've had to replace my cap a few times to get it in the "zone".
I also use a 13# but some catalogs list a 14# original. If you go to a higher cap remember to make sure your hoses are able to handle the extra pressure. I will be moving to a higher cap since it has been getting warm lately.
I may be rumdum from cleaning solutions (helped the wife clean the salon today) but I thought all our radiator caps were vented, which provides a safe, closed method of directing coolant overflow into an external reservoir and then, once the engine has cooled and everything has contracted, the return of the exhausted coolant back into the engine.
Essentially, it's a simple double gasket, double spring arrangment which provides a closed path both under pressure and under vacuum...
Or, am I missing something? *G*
The only thing I can think of is the old-style lever vent caps where you could manually vent the radiator pressure before removing the cap...
In the old days, there was simply a cap on the radiator (screw-on generally) which had a puke hole in it, hence the pictures of the old Model A's steaming away on the side of a long grade....when I would drive to parades, as a child, with a gentleman who's '13 Buick touring I helped maintain, I was the designated valvetrain oiler (exposed valvetrain) and radiator filler. I learned quickly about handling hot water and hot radiators safely....and keeping my fingers out of the valvetrain while the engine was running *G*